The present invention relates generally to signal acquisition probes and more particularly to a time shifting signal acquisition probe system.
Digital oscilloscopes generally acquire electrical signals from a device under test via signal acquisition probes coupled to input signal channels of the oscilloscope. Each input signal channel has acquisition circuitry which digitizes the received electrical signal and stores the resulting digital data in a circular buffer. The electrical signals are also coupled to trigger circuitry for generating a trigger signal. In its simplest form, the trigger circuitry is set-up by a user using front panel controls on the digital oscilloscope to trigger at a certain threshold level on a rising or falling portion of the electrical signal on one of the input signal channels. The electrical signals on the other input signal channels trigger off of the electrical signal on the assigned triggered channel. The digital oscilloscope also has controls for setting a hold-off or pre-trigger time. The hold-off or pre-trigger time allows the for the accumulation of digital data prior to a trigger signal. The trigger circuit is armed after the hold-off time and the next occurrence of the electrical signal that matches the trigger parameters results in the generation of the trigger signal. Each of the respective circular buffers continues to receive digital data from their respective acquisition circuits until a post trigger time has expired at which time the digital data in each of the circular buffers is transferred to a reference storage memory as a waveform record. Display circuitry retrieves the waveform records from the reference storage memory and formats the digital data for each of the waveform records and displays a portion of the waveform records on a display device, such as a cathode ray tube or a flat panel display.
Digital oscilloscope, such as the TDS5000B Series Real Time Digital Oscilloscopes, manufactured and sold by Tektronix, Inc., Beaverton, Oreg., are factory calibrated so that propagation delays in the various input signal channels and the trigger circuitry are accounted for. A common signal is applied to each of the input signal channels. One of the input signal channels is designated as a trigger reference signal channel. Waveform records are acquired for each of the input signal channels using the trigger signal from the trigger reference signal channel. The time location of the threshold crossing of the trigger event in the reference waveform record is compared to the time location of corresponding threshold crossings in the other waveform records from the other input signal channels. The differences between the threshold crossing times of the other input signal channels to the threshold crossing of the reference channel is calculated. The leading or lagging time differences of the threshold crossings are stored as calibration values for each of the input signal channels. The resulting calibrated oscilloscope has a calibration plane at the front panel connections of each of the input signal channels. That is, a common signal applied to each of the input signal channels will result in a substantially time aligned digital data in the waveform records. Further, display of the digital data from the waveform records on the digital oscilloscope will be substantially time aligned.
When signal acquisition probes are connected to the input signal channels of the measurement test instrument, the acquired signals may not line-up due to propagation delays existing in signal acquisition probes. This results in what is called signal skewing. Various systems and methods have been developed to deskew the measurement probes to realign the signal from the various input signal channels.
The TDS5000B Series Real Time Digital Oscilloscopes include channel-to-channel deskew capabilities for aligning signals from measurement probes. Voltage and/or current probes are connected to a deskew fixture, such as the 067-1478-00 Power Measurement Deskew Fixture, manufactured and sold by Tektronix, Inc., that provides time aligned output signals. The deskew fixture is coupled to a signal source, such as the AUX OUT output of the TDS5000B oscilloscopes. A probe deskew algorithm is activated in the oscilloscope which allows a user to manually adjust the time positions of displayed waveforms of the acquired signal from the measurement probes. Generally, the displayed waveform of the input signal from the triggered input signal channel is used as a reference and the displayed waveform of the other input signals from the other signal channels are time aligned with the displayed reference waveform. The deskew algorithm allows a user to select an input signal channel to deskew and provides controls for positioning the displayed waveform from the selected input signal channel. As the displayed waveform from the selected input signal channel is time aligned with the displayed reference waveform, the deskew algorithm determines the time difference between the current time position of the displayed waveform and the starting time position of the waveform. Once the selected displayed waveform is time aligned with the displayed reference waveform, the deskew algorithm allows the user to save the time difference value of the selected displayed waveform as the deskew value for the signal acquisition probe and input signal channels combination. Each of the other signal acquisition probe and input signal channel combinations are deskewed in the same manner with the time difference values saved as the deskew values. The signal acquisition probes are removed from the deskew fixture. Because of the deskew operation, any time offset of the signals from the device under test will be as a result the time offsets of the signals in the device under test and not from the propagation time delay differences in the signal acquisition probes.
The TDSPWR3 Power Measurement and Analysis Software, manufactured and sold by Tektronix, Inc., Beaverton, Oreg., includes static deskew algorithms that automatically sets the probe deskew based on the probe type supported. The deskew algorithms contain tables of probe types and nominal skew time values for each of the probe types. The static deskew algorithms include a user interface where a user selects the probe type of the probe connected to a selected “FROM” input signal channel and the probe type of the probe connected to a selected “TO” input signal channel. The user presses a run button and the static deskew algorithms repositions the “TO” input signal channel to the “FROM” input signal channels by the amount of the deskew time value of the probe connected to the “TO” input signal channel.
The TDS8200 Digital Sampling Oscilloscope, manufactured and sold by Tektronix, Inc., Beaverton, Oreg., has multiple slots or bays for receiving various electrical modules for measuring electrical or optical signals. Each slot has an electrical interface that provides electrical power, communications and signal transport to and from the electrical modules. The 80E00 Series of electrical Sampling Modules is one set of modules usable with the TDS8200 Digital Sampling Oscilloscope. The sampling modules samples and digitizes a signal under test in response to a repetitive strobe signal and couples the digitized samples to the oscilloscope. Optional 80E00 Series Sampling Module Extenders may be used to place the electrical module outside the oscilloscope bay to avoid input signal degradation that can occur when using interconnect cables between the sampling module and the device under test. There is a one meter extender cable having a nominal +5 ns propagation delay and a two meter extender cable having a nominal +10 ns propagation delay. Each electrical interface has two lines dedicated to monitoring the status of the electrical module in the oscilloscope. The status lines are either open or shorted and indicate if an extender cable is present and, if so, the length of the extender cable connected to the electrical module. The oscilloscope communicates with the electrical module requesting the states of the status lines. The electrical module monitors the state of the status lines and communicates the states back to the oscilloscope. Software in the oscilloscope includes a table of status line states and delay values that is accessed to modify the instrument display horizontal position display setting by the amount of the extender cable delay.
Currently, deskewing signals from various measurement probes requires the use of a deskew fixture or deskew algorithms that store specific nominal deskew time values for specific measurement probes in a table. In the former case, a user needs to purchase a deskew fixture in order to deskew signals. In the latter case, the user is restricted to the specific probe and deskew time values stored in the in the deskew algorithm table. A user is required to upgrade the deskew algorithm with a new table each time a new probe is introduced for the Power Measurement and Analysis Software.
What is needed is signal acquisition probe system that does not require the use of a deskew fixture for time aligning electrical signals from various signal acquisition probes. Further, the signal acquisition probe system should facilitate the introduction of new signal acquisition probes without the need for upgrading software to deskew the probe. Additionally, the signal acquisition probe system should provide a user with a display indicating the time the electrical signal is acquired at the device under test measurement point relative to the measurement test instrument trigger signal.